Tomographic images and the like of subjects, taken by a variety of devices such as the X-ray CT (computed tomography) machines and the MRI (magnetic resonance imaging) machines, have been conventionally known as images to be used for medical diagnostic purposes. Meanwhile, the computer-aided detections (hereinafter referred to as CADs) have been developed for: analyzing the aforementioned medical images with use of computers; detecting lesion candidates from shadows in the medical images; and providing a doctor with the detected lesion candidates. The CADs automatically detect regions expected as lesions in the images (hereinafter referred to as lesion candidate regions) based on shape properties and concentration properties of the lesions for reducing the burden of doctors.
For example, some of lesion candidates (e.g., polyps in regions of the large intestine) are formed in spherical shapes and thus have unique shape properties. In Patent Document 1, for instance, a curvature value typified by the shape index or the like is calculated as a feature amount indicating a shape property and abnormal shadow candidate regions are narrowed down based on the shape of a curved surface indicating concentration distribution of an image. In Patent Document 2, on the other hand, features indicating abnormality in a scanned image are highlighted and/or displayed comparably with the original scanned image as a user interface of a CAD for enhancing convenience of an operator.
Meanwhile, a technique has been developed for generating an image displaying the inside of a hollow organ developed about the axis of the hollow organ (hereinafter referred to as an panoramic image) as an image displaying method for actively diagnosing the insides of the hollow organs such as the large intestine (Patent Document 3). The panoramic images are advantageous in that doctors and the like can easily find lesion candidates because the entire surface of the hollow organ inside is viewable simultaneously. Further, a technique has been developed for generating a virtual endoscope image based on volume image data organized by the accumulation of plural sheets of tomographic images obtained by the aforementioned devices such as the X-ray CT devices (Patent Document 4). A virtual endoscope image is displayed by a method of: irradiating a projection object with a virtual ray from a virtual point-of-view set in the inside of a hollow organ; extracting a voxel with a brightness value greater than or equal to a predetermined threshold from voxels arranged on a line-of-sight; and projecting the extracted voxel on a projection surface. Similarly to an image obtained by an endoscope, the inside of an organ is observable with the virtual endoscope image (Patent Document 4).